An Experimental and Computational Study - American Chemical Society

Article pubs.acs.org/JPCC

Noble Gas Adsorption in Copper Trimesate, HKUST-1: An Experimental and Computational Study Zeric Hulvey,†,‡,§ Keith V. Lawler,§ Zhiwei Qiao,∥,⊥ Jian Zhou,⊥ David Fairen-Jimenez,∥,# Randall Q. Snurr,∥ Sergey V. Ushakov,∇ Alexandra Navrotsky,∇ Craig M. Brown,†,○ and Paul M. Forster*,§ †

Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States § Department of Chemistry, University of Nevada Las Vegas, Box 454003, Las Vegas, Nevada 89154-4003, United States ∥ Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208-3120, United States ⊥ School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab for Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, Guandong, China # Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB2 3RA, United Kingdom ∇ Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, California 95616, United States ○ Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ‡

S Supporting Information *

ABSTRACT: A joint experimental and computational study of noble gas adsorption in the metal−organic framework (MOF) material HKUST-1 has been carried out. Using a standard gas adsorption analyzer fitted with a cryostat, isotherms were measured for Xe, Kr, Ar, and Ne at optimum temperatures for the determination of loading-dependent heats of adsorption using the Clausius−Clapeyron equation. Direct calorimetric measurements for Kr and Xe adsorption provide comparable heats of adsorption. A detailed analysis of the experimental data alongside complementary grand canonical Monte Carlo (GCMC) simulations led to the conclusion that the strongest binding for noble gases occurs in and around the small tetrahedral pockets and not at the accessible Cu(II) sites in the structure. Synchrotron X-ray and neutron powder diffraction experiments with in situ gas loading confirm the assignment of preferred binding sites inferred from the adsorption measurements and simulations.

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INTRODUCTION Metal−organic frameworks (MOFs) have shown significant promise as materials capable of performing many types of gas separations.1,2 One of the most interesting and well-studied MOFs is the copper trimesate structure most commonly referred to as HKUST-1, first reported by Chui et al. in 1999.3 Over the past decade, it has been shown to be effective in a number of gas separation applications, including the removal of CO2 from air or flue gases;4,5 the separation of H2 from CO2, CH4, and N2;6 ethylene−ethane separations;7 and propylene− propane separations.8 Its synthesis has been refined to the point where phase-pure samples can be easily synthesized solvothermally at relatively low temperatures without Cu2O impurities that are present in higher-temperature preparations.9 Parameters to ensure complete activation of the entire void space have also been presented, resolving a wide discrepancy in past surface area and pore volume data.3,10−12 The structure of HKUST-1 (Figure 1), consisting of copper(II) “paddlewheels” connected through 1,3,5-benzenetricarboxylate ligands, contains many attributes which make it © 2013 American Chemical Society

an ideal material to study gas-sorbent interactions. Each Cu atom contains an axial water molecule that can be removed to leave an open coordination site. The resulting complex pore system is composed of three different types of connected cavities. The first is located at the center of the unit cell and has a diameter of approximately 11 Å. Its surface is decorated predominantly by benzene rings from the ligands facing toward the cage surface. This is connected to another slightly larger cage centered on the faces of the unit cell with a diameter of nearly 13 Å. The open Cu(II) sites decorate a large fraction of its surface. Small, tetrahedrally shaped cavities (subsequently denoted “pockets”) with a diameter of approximately 5 Å are accessible through triangular windows (diameter approximately 4 Å) from the largest pore. The interior surface of the pockets consists of a tetrahedral arrangement of four benzene rings from the ligands. In short, the material simultaneously contains small and large pores, coordinatively unsaturated metal sites, Received: September 3, 2013 Published: September 20, 2013 20116

dx.doi.org/10.1021/jp408034u | J. Phys. Chem. C 2013, 117, 20116−20126

The Journal of Physical Chemistry C

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We are interested in Kr/Xe separations using pressure swing adsorption. While this project is driven primarily by Department of Energy (DOE) needs for spent nuclear fuel processing, an improved separation technique could also provide substantial savings for commercial production of these gases. Both Kr and Xe are extremely rare and produced as a minor byproduct of N2/O2 cryodistillation; consequently, worldwide production is effectively fixed. High cost, particularly for Xe, limits its use beyond lighting. Given its remarkable anesthetic properties,21,22 the high cost of the gas is unfortunate. A material with exceptional selectivity for Xe may lead to an alternative commercial process for direct capture from the atmosphere. A few reports have shown that HKUST-1 is a promising material to selectively adsorb Xe over Kr. Using breakthrough experiments, Mueller et al.23 showed in 2006 that HKUST-1 could selectively adsorb Xe over Kr to the point where a mixture of 94% Kr and 6% Xe could be reduced to a Xe concentration below 50 ppm. This selectivity was not attributed to any specific feature of the structure. Farrusseng et al.11 measured isosteric heats of adsorption at very low coverages for Kr and Xe using pulse−response experiments and obtained values of approximately 9 and 20 kJ/mol, respectively. Very recently, Dorcheh et al.24 published heats of adsorption along with temperature−programmed desorption plots for Kr and Xe in HKUST-1, with values at initial loading of approximately 18 and 27 kJ/mol, respectively. Another report showed by means of 129Xe NMR spectroscopy that when fully activated, adsorbed Xe can occupy two different sites in HKUST-1. However, no additional structural information could be inferred.25 A recent computational study26 predicted that Kr and Xe bind primarily inside the tetrahedral pocket, but this result was not conclusive because the force fields used in the simulation do not include any polarization or other strong interactions with the open Cu(II) sites. Because crystallographic evidence shows that the metal site in HKUST-1 does bind methane, a nonpolar adsorptive whose diameter and polarizability are nearly identical to those of Kr, the question remains as to whether the metal site would cause a similar enhancement with noble gases. Studies of Kr and Xe adsorption in other MOFs have also not definitively clarified which structural features result in favorable adsorption or separation properties. A study of monohalogen-substituted MOF-5 analogs reported a correlation between increasing ligand polarizability and heats of adsorption for Kr and Xe;27 however, these materials exhibited quite low heats of adsorption and only moderate Xe/Kr selectivities. A high-throughput computational study of over 137,000 hypothetical MOFs by Sikora et al.28 predicted that Xe/Kr selectivities are greatest in structures containing uniform pore sizes just large enough to accommodate one Xe atom. A combination of experimental and simulated data has also shown favorable Xe/Kr selectivity in the small pore material MOF505.29 On the other hand, experimental data have been reported which show high Xe capacity and high Xe/Kr selectivity in Ni-MOF-74, a structure containing a very large channel with a high concentration of open metal sites.30,31 Additionally, maximum entropy methods applied to X-ray powder diffraction data combined with quantum mechanical calculations have indicated preferential Xe binding in the larger of the two pores in the structure of MFU-4l.32 While these previous studies clarify adsorption behavior for Kr and Xe in several benchmark MOFs, there has not been a

Figure 1. A view centered on the three main pores in HKUST-1. Yellow and red spheres indicate the 11 Å pore and the 13 Å pore decorated with Cu(II) sites, respectively. A green sphere has been placed in one of the tetrahedral pockets (between and above the red and yellow spheres).

and relatively flat surfaces from the organic ligands. However, as much of the gas adsorption work in the MOF field is dominated by H2 and CO2 adsorption, HKUST-1 has become most notable for its open metal sites and to a much lesser extent for the fact that it contains a diverse array of pore sizes. Gas adsorption behavior in HKUST-1 has been well-studied, including a few neutron diffraction studies to resolve binding sites for various gases. Peterson et al.13,14 have shown on two separate occasions that D2 molecules occupy up to nine different sites depending on loading levels, with the Cu(II) sites filling up first, followed by sites in the pocket, and subsequent complete filling of the remaining pore space. Acetylene was also shown to preferentially occupy the Cu(II) sites, followed by occupation of a site near the windows to the pocket.15 Similarly, diffraction studies of CD4 adsorbed in HKUST-1 also found adsorption occurs at the Cu(II) sites, at the pocket center, and at the pocket windows.16,17 Two joint computational and experimental studies by Karra and Walton18,19 have highlighted that the competing influence of Cu(II) sites and small pores cause HKUST-1 to display no adsorptive selectivity for CO/ CH4 mixtures and that the existence of both structural features can enhance CO2 adsorption, even as most crystallographic evidence described above points to the Cu(II) sites dominating adsorption at low loadings. The heat of adsorption of gases on MOF structures represents one of the critical parameters necessary for assessing separation potential.20 Isosteric heats of adsorption (termed “heats”) are usually determined by applying the Clausius− Clayperon equation to isotherms collected at multiple temperatures. For ideal gases, isosteric heats of adsorption (positive by definition) are equal in absolute values to differential enthalpies of adsorption, which are exothermic and can be measured by calorimetry. It is now becoming common for heats of adsorption for H2 and (more recently) CO2 to be reported for MOFs with promising adsorption characteristics. While this choice of gases is in part due to the importance of H2 storage and CO2 removal from flue gas streams, they are also unusually well-suited for heat of adsorption measurements. For H2, isotherms can be collected at 77 and 87 K (liquid N2 and Ar temperatures) relatively easily. Isotherms to determine CO2 heats of adsorption are typically measured in the 273323 K range, which can be reached with an ice bath and conventional heaters. Heats of adsorption are rarely determined for other gases, such as Kr and Xe, which require the measurement of adsorption isotherms in less easily accessed temperature ranges. 20117



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fully activated samples of HKUST-1 (see Supporting Information for details).9 Once the sample was assembled inside the sample well of the cryostat, approximately one hour was given to allow for thermal equilibration of the sample and cryostat. Gas adsorption isotherms were then collected at the following temperatures for the respective gases: 240, 260, and 280 K for Xe; 180, 200, and 220 K for Kr; 120, 140, and 160 K for Ar; and 40, 50, and 60 K for Ne. The isotherms were collected over the range of 0 to 600 mmHg with a specified gas dosing amount (which was varied for different gases and different temperatures) and long equilibration times at each volume increment. Type I isotherms were observed in all cases. As hysteresis in these isotherms would indicate that equilibrium had not been reached, we measured desorption isotherms in all cases and used only data exhibiting essentially no hysteresis. After each isotherm measurement, the isolation valve on the sample tube fitting was closed, the tube was removed from the cryostat and heated to 100 °C under vacuum for 1−2 h to remove all of the adsorbed gas before another measurement was taken. Heat of adsorption calculations for each gas were carried out by applying the Clausius−Clayperon equation to the isotherms collected. Plots of all isotherms are included in the Supporting Information. Calorimetric Measurements. Enthalpies of adsorption for Xe and Kr on HKUST-1 at low coverages were also measured directly by gas adsorption calorimetry using a Micromeritics ASAP 2020 instrument combined with a Setaram Sensys Calvet-type calorimeter. The experimental setup is described in detail elsewhere.35 Approximately 40 mg of HKUST-1 (samples obtained from the same synthesis batch used from the gas adsorption measurements) were pelletized and placed in one side of a fork tube with a total volume of 10 mL. The tube was inserted into the calorimeter chamber and connected to the analysis port of the ASAP 2020. Validity of factory electrical calibration of calorimeter sensitivity in the configuration used was checked by melting a Ga standard in a fork tube. Sample activation was performed using the conditions described above for isotherm measurement. Typical heat flow traces from these experiments are shown in the Supporting Information (Figure S8). For measurement, the ASAP 2020 was operated in incremental dosing mode with a dose amount of 2040 μmol and an equilibration delay of 0.5 h. The calorimeter was operated isothermally at 25 °C; fluctuation during all experiments did not exceed 0.005 °C. Integration of calorimetric peaks using a linear baseline gave heat effects in the range of 50150 mJ. Dividing the values by the corresponding individual dose quantity recorded by the ASAP 2020 provides the heat of adsorption per dose. Typical quantities adsorbed were 36 μmol per dose. Because the gas expands in both the sample and the empty reference cell of the calorimeter, the measured heats are enthalpies of adsorption. The free space was measured with He after adsorption experiments to prevent possible contamination of the pores with He gas. Two experiments were performed for Xe adsorption and three experiments for Kr adsorption with reactivation of the sample at 180 °C performed between the experiments. Enthalpies of adsorption of first doses were reproducible within 12 kJ/mol, however significant scatter in data was observed at higher coverage. Simulations. Gas adsorption on HKUST-1 was investigated using grand canonical Monte Carlo (GCMC) simulations,36 performed with our in-house multipurpose code RASPA.37 In the grand canonical ensemble, the chemical potential, volume,

study that provides significant insight into the mechanism of noble gas adsorption and separation. Definitive knowledge of binding sites for Kr and Xe is essential to both explain observations of separation potential in certain MOFs and direct future research into better materials for this application. In this report we present a joint experimental and computational investigation of noble gas binding in HKUST-1 that clarifies the mechanism of its selectivity for Xe over Kr. Carefully measured heat of adsorption data, obtained both from adsorption isotherms and by direct gas adsorption calorimetry, coupled with GCMC simulations provide a remarkably consistent atomistic picture of noble gas sorption in this material. These two different experimental approaches, as well as GCMC calculations, yield very similar heats of adsorption which are appreciably different from some existing literature values. Together, the data indicate that the pockets are much more important for noble gas adsorption than the open Cu(II) sites. Finally, we present structural data from synchrotron X-ray diffraction and neutron diffraction experiments on gas-loaded HKUST-1 samples that confirm preferential binding of noble gases inside and around the pockets and not at the open Cu(II) sites.

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METHODS Gas Adsorption Measurements. Gas adsorption isotherms were measured using a Micromeritics ASAP 2020 instrument fitted with a helium cryostat manufactured by ColdEdge technologies.33 The cryostat interfaces with the analysis port on the analyzer through a specially designed joint with an isolation valve which enables removal of the tube from the cryostat without exposing the sample to the atmosphere. Samples are loaded inside glass tubes which are attached to this joint and fitted inside a sample well area of the cryostat. The inside of the sample well is purged with a constant positive flow of He to prevent condensation of gases on or around the sample tube and to enable efficient heat transfer to the sample tube. The bottom bulb of the sample tube is surrounded by aluminum heat shields in order to ensure a large region of stable temperature control around the sample. A LakeShore model 336 temperature controller provides stability of better than ±0.01 K within the range of 25−350 K. Actual temperatures in the sample tubes (which inherently differ from those measured by the temperature controller) are determined to high precision through a calibration curve constructed by the measurement of condensation pressures of various gases using the ASAP 2020 instrument. Freespace calibrations for specific sample tubes are performed by measuring blank tube isotherms at 20 K intervals over the entire temperature range of the measurements. This removes the need for the typical He freespace measurement at the beginning of the isotherm, which could result in He contamination. The volume occupied by the sample was subtracted based on the sample mass and the density calculated from the crystal structure. In a typical measurement, about 130 mg of the as-synthesized HKUST-1 material was introduced into a calibrated sample tube and activated under vacuum at 180 °C for 18 h. As expected, the material changed from bright blue to deep purple. The activated sample mass was then determined by reweighing the entire sample tube. A Brunauer−Emmett−Teller (BET) surface area of 1684 m2/g was calculated from the N2 adsorption isotherm at 80 K using the methods described by Walton and Snurr34 and is comparable to those reported for 20118



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Figure 2. Experimental heats of adsorption for Ne, Ar, Kr, and Xe in HKUST-1 determined from adsorption isotherms.

and temperature are kept fixed as in adsorption experiments. The chemical potential was related to the system pressure by the Peng−Robinson equation of state. In the simulation, molecules were randomly moved, inserted, and deleted, which allows the number of molecules in the framework to fluctuate. A total of 50,000 equilibration cycles and 250,000 production cycles were used for each simulation, where a cycle includes N Monte Carlo moves, with N being the number of molecules in the system. The isosteric heat of adsorption was obtained by means of the fluctuation theory.38,39 An atomistic model was used for HKUST-1, where the framework atoms were kept fixed at their crystallographic positions. Details regarding the parameters used are included in the Supporting Information. The standard 12−6 Lennard− Jones (LJ) potential was used to model the dispersive and repulsive interatomic interactions, with a cutoff of 12 Å. The Lorentz−Berthelot mixing rules were employed to calculate fluid/framework parameters. The LJ parameters for the framework atoms were obtained from the Dreiding force field,40 and if not available in Dreiding (i.e., Cu), from the Universal Force Field (UFF).41 Tables of these parameters are included in the Supporting Information. Quantum diffraction effects were tested for Ne adsorption at 40 K using Feynman− Hibbs corrections.42,43 Powder Diffraction Measurements. Synchrotron X-ray powder diffraction (XRPD) data for Kr- and Xe-loaded samples of HKUST-1 were measured at the Advanced Photon Source at Argonne National Laboratory on the 17-BM materials diffractometer (λ = 0.7291 Å). Activated samples of HKUST1 were loaded into glass capillaries inside a nitrogen glovebox. After data were collected on the bare samples, they were dosed with known quantities of Xe or Kr using a custom built gas dosing manifold of known volume. Dosing and measurement temperatures ranged from 260 K at low loading to 240 K at higher loading for Xe, and 200 K at low loading to 140 K at higher loading for Kr. Neutron powder diffraction (NPD) data for an Ar loaded sample of HKUST-1 were measured on the high-resolution BT1 diffractometer at the National Institute of Standards and Technology Center for Neutron Research (NCNR) with a Ge(311) monochromator (λ = 2.078 Å) and in-pile collimation

of 60′. An activated sample of HKUST-1 (1.6711 g) was loaded into a cylindrical vanadium can inside a helium glovebox. After data on the bare sample was collected at 8 K, the sample was dosed at 100 K with a known volume of Ar corresponding to roughly 1 Ar atom per tetrahedral pocket using a custom built gas dosing manifold of known volume. Following complete adsorption of the Ar dose the material was cooled to 8 K for data collection. All XRPD and NPD data were analyzed using the Rietveld method as implemented in EXPGUI/GSAS.44,45 The model of the bare structures were initially refined and used as the starting point for subsequent refinements of the gas-loaded samples. Fourier difference maps were generated to determine missing electron or nuclear density resulting from the adsorbed gas atoms, and their locations and occupancy were subsequently refined. Results of all structure refinements are included in detail in the Supporting Information.

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RESULTS AND DISCUSSION Experimental heat of adsorption curves were determined for Ne, Ar, Kr, and Xe by collecting gas adsorption isotherms in appropriate temperature ranges and applying the Clausius− Clayperon equation (Figure 2). For accurate heat of adsorption determination, it is important to ensure hysteresis is not present in the adsorption isotherms; all isotherms are plotted in the Supporting Information and show no hysteresis. The heats of adsorption for Kr and Xe in Figure 2 agree well with those reported by Soleimani Dorcheh et al. very recently,24 but differ considerably from those measured by pulse−response experiments (with a difference of more than a factor of 2 in the case of Kr).11 Because our gas adsorption studies and literature pulse− response measurements provided contradictory initial heats of adsorption, we carried out calorimetric studies using an ASAP 2020 gas adsorption analyzer to dose gases into a Setaram Sensys Calvet-type calorimeter. Room-temperature measurements avoid complications of water vapor condensation outside the tube and minimize temperature differences between the sample and incoming gas. However, operating at room temperature limits how much gas can be loaded onto the 20119



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sample. Results are plotted in Figure 3. Enthalpies of adsorption of Kr and Xe at low coverage from calorimetric

Figure 4. Comparison of experimental (blue) and simulated (red) heats of adsorption for Ar (top) and Ne (bottom).

identical loadings. Both experimental and simulated heat of adsorption curves increase by several kilojoules per mole at still higher loadings in the case of Xe and show slight increases for Kr and Ar. Such increases in heat of adsorption at higher loadings, while not intuitive, have been noted in other materials and are attributed to gas−gas interactions as pores fill with gas atoms or molecules.46 The simple Lennard−Jones and Coulomb potential typically used in GCMC simulations does not capture complicated interactions such as polarization or transfer of electron density between adsorbed gas molecules and coordinatively unsaturated metal sites. As noble gases are neutral, Coulomb interactions were turned off entirely in this work. Because accessible Cu(II) sites are present in HKUST-1 and are known to be the dominant adsorption site for H2 and other gases, the close reproduction of our experimental heat of adsorption curves by GCMC simulations was not necessarily expected and suggests that the Cu(II) sites are not the primary adsorptive sites. The GCMC simulations provide an ensemble-averaged picture of gas density in the pores. The simulations for Xe (Figure 5) show initial adsorption in the pockets followed by adsorption at the pocket windows before the final saturation of the pore volume. Not surprisingly, the simulations show no significant density around the coordinatively unsaturated Cu(II) sites. In HKUST-1, there are six accessible Cu(II) sites and four pocket windows for every pocket. Complete filling of the pocket with one noble gas atom would correspond

Figure 3. Comparison of calorimetry (black points) with heats of adsorption determined from isotherm measurement (blue) or GCMC simulations (red) for Xe (top) and Kr (bottom).

experiments are −26 ± 1 and −18 ± 1 kJ/mol for Xe and Kr, respectively. Both of these values are approximately 2 kJ/mol lower than isosteric heats at low loadings. This variation is close to experimental uncertainties, and in the case of Xe, the agreement continues up to moderate loadings. The reason for the more rapid decrease in the case of Kr is unclear. We note that it coincides with the large spread in the data between experiments (Figure S8 of the Supporting Information), which may be related to increasing uncertainty in measurements of small changes in pressure and dose amount calculations. The measured heats of adsorption for Kr and Xe also agree well with previously published results from GCMC simulations.26 To investigate further, we carried out a new set of GCMC simulations at temperatures corresponding to those used here experimentally and extended the work to include Ar and Ne. Other than the temperature choice, we used an identical approach to that published previously. The calculations were performed without sharing of the experimental data to eliminate the possibility of unintentional bias. As shown in Figures 3 and 4, the GCMC simulations match our experimental heat of adsorption curves closely in most respects. Specifically, they predict an initially high heat of adsorption that decreases quickly with further loadings. Remarkably, both simulation and experiment show the decrease occurring at 20120



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Figure 5. (a) Accessible volume of HKUST-1 as an aid in interpreting the simulated density distribution maps for Xe adsorption, with the surface plotted representing the van der Waals surfaces for the framework atoms. In image (b), three different Xe loadings are indicated; density distribution maps are provided for these loading levels (c−e). Plot (c) represents Xe density at loading A on the isotherm; deeper shading represents higher configuration density and therefore a higher probability that adsorption will occur at that site. Plots (d) and (e) correspond to loadings B and C, respectively.

to an uptake of 0.83 mmol/g, whereas one atom occupying each pocket window or Cu(II) site would correspond to approximately 3.3 or 5.0 mmol/g, respectively. Figure 6 reproduces the experimental Xe heat of adsorption data with three vertical bars; the left bar represents complete filling of the pockets, the middle bar complete filling of pocket windows, and the right bar complete saturation of Cu(II) sites. Filling of the pockets matches the decrease in the heat of adsorption curves strikingly well; no features are obvious that correspond to

saturation of pocket windows or Cu(II) sites. Our careful activation and high measured nitrogen surface areas make partial solvation of Cu(II) sites unlikely. Comparable results in the case of Kr further support this conclusion. In addition to Xe and Kr, we also studied Ar and Ne adsorption (Figure 4). While the agreement in absolute terms between simulation and experiment is less impressive than that for Kr and Xe, values are still within approximately 2 kJ/mol at the lowest loadings. One possible reason for the larger discrepancy between simulation and experiment is quantum effects, particularly in the case of Ne. This has been noted previously in studies of He and Ne adsorption in carbon nanotubes.47 However, additional calculations with Feynman− Hibbs corrections did not result in a difference in uptake or heat of adsorption values. We attribute the differences with experimental values in this case to force field uncertainties. The GCMC simulations do an excellent job of predicting the trends for the heat of adsorption curves, with a gradual decrease in values occurring over the same loading levels. Interestingly, the decrease in heat of adsorption for both Ne and Ar occurs at roughly four times the loading that it occurs for Kr and Xe (Figure 6). This behavior is easily attributed to preferential adsorption at the four windows leading into the pockets instead of at the pocket center. Density distribution maps from the GCMC simulations (Figure 7) indeed show the initial adsorption of Ar at the pocket windows. Our simulation results are in line with an earlier GCMC simulation study of Ar sorption in HKUST-1.48 Heats of adsorption for Ar have also been measured previously by similar methods.49 The initial heat of adsorption at lowest loadings is very close to our value (14.0 versus 14.2 kJ/mol, respectively); but their data show a decline in heat of adsorption at significantly lower loadings. The most

Figure 6. Experimental heat of adsorption data for Xe (red, left axis) and Ar (blue, right axis) with vertical dashed black lines denoting amount of adsorbed gas representing complete saturation of the pockets (0.83 mmol/g), pocket windows (3.3 mmol/g), and Cu(II) sites (5.0 mmol/g). 20121



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The structure determined for the lowest loading of Xe (0.357 mmol/g) confirms that the primary adsorption occurs in the center of the pocket, placing the Xe approximately 4.5 Å from the center of the four benzene rings defining the pocket wall (Figure 8). For higher loadings, a secondary binding site appears at the windows leading into the pockets (the “window site”), formed by three trimesate ligands. Occupation of this site begins before the pocket site has completely filled (Figure 9). The shortest Xe−framework contact for the window site is with the three ligand hydrogen atoms. In fact, the Xe atom is closer to this hydrogen atom, a ligand carbon atom, and the carboxylate oxygen atom than it is to the Cu(II) site. This confirms that the main adsorptive interactions are with the ligand components of the window and that the Xe atoms are not interacting in a significant way with the Cu(II) atoms. While the atomic displacement parameters (ADP), reflecting the thermal motion and potential site disorder, are relatively large, there is a gradual change in the fractional coordinate of the window Xe atom. The window Xe moves toward the center of the pocket with higher loadings, decreasing the Xe−Xe distance from 6.04(3) Å to 5.19(1) Å. Once adsorption at the window site is observed, both the pocket and window sites populate at similar rates for additional doses (Table 1). Partial occupation of the secondary, lower-energy window site prior to full occupation of the pocket site correlates with the experimental heat of adsorption data (Figure 10, left). Heat of adsorption values are very high in the region of dose 1, where adsorption is observed exclusively in the pocket. The sharp decrease in the measured heat of adsorption begins as the window sites begin to occupy and eventually levels off around dose 5 when the pocket site is almost fully occupied. This structural data is also consistent with the positions predicted by the GCMC simulations (Figure 5c corresponds to loading levels near dose 5). Additional powder patterns were collected for expected loadings of 2.5 mmol/g of Xe and higher where lower energy sites were expected to be occupied. However, a significant increase in background and decrease in peak intensity makes XRPD structure refinement ambiguous as electron density contrast between the framework and Xe in the pores is reduced. The absence of further specific sites in the

Figure 7. Adsorbed gas density distribution maps for Ar at low loadings based on GCMC simulations showing primary adsorption sites in the pocket windows.

likely explanation for this difference is a partial blockage of the tetrahedral pockets in their sample, which is consistent with their appreciably lower measured BET surface area. To confirm assignment of preferred binding sites inferred from the correlation of experimental adsorption data and molecular simulations, we carried out synchrotron X-ray and neutron powder diffraction experiments on samples of HKUST-1 loaded with noble gases. Rietveld refinements were performed on data for both the bare framework and the material dosed with various amounts of each gas after locating the adsorbed atoms using Fourier techniques. Values of refined site occupancy factors (SOF) for the various adsorption sites are converted to volumetric quantities for easier comparison of the structural data to the heat of adsorption curves. For all three gases, there is no evidence of any binding at the open Cu sites; all distinct binding sites are within and around the small pockets.

Figure 8. View of HKUST-1 showing the strongest binding site for Xe (orange atoms) at the pocket center, with two views of the small pocket isolated (top left and bottom left) and view of one unit cell down the c-axis (right). 20122



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Figure 9. View of HKUST-1 showing secondary Xe binding site (orange atoms) at the four windows to the small pockets as well as the pocket site (one pocket isolated on the left; view of one unit cell down the c-axis on the right).

smaller van der Waals radius, Kr does not experience as substantial an interaction with the surface of the pocket while at the center. The pocket site therefore changes from a single crystallographic site to four that are symmetry-related, only one of which may be occupied. A much closer Kr−framework distance in a corner of the pocket is thus possible (minimum Kr−C distance of 3.95(2) Å). This illustrates one reason why the heat of adsorption for Kr at the lowest loadings is considerably lower than that for Xe; the Xe atom at the center of the pocket interacts with the benzene rings from all four ligands, while the Kr atom is interacting with only three. Occupation of the secondary window site occurs in the same volumetric uptake region with similar relative fractional occupancies of the sites for Kr as is observed for Xe (Table 2). The window site is similarly at the center of the window (Figure 11, right) with the shortest framework contact at the ligand H atoms. In dose 2, the Kr−Kr distance is 4.40(3) Å. In dose 3, the window site moves closer to the center of the pocket to the extent that occupation of the window site does not allow simultaneous occupation of all four symmetry related pocket sites. At this loading, the Kr pocket site can either be modeled at the center of the pocket with a very large ADP value or be split into the four disordered sites slightly off-center with a more reasonable ADP value. We have refined the structure using the latter model, where the Kr−Kr distance of 4.63(2) Å represents the distance from the window site to the

Table 1. Refined SOFs for Xe Sites As a Function of Loading Amountsa dose

Xe adsorbed per Cu

Xe adsorbed per pocket

Xe adsorbed (mmol/g)

pocket site SOF

window site SOF

1 2 3 4 5

0.072(1) 0.160(2) 0.187(2) 0.207(2) 0.315(2)

0.432(4) 0.96(1) 1.12(1) 1.24(1) 1.89(1)

0.357(3) 0.79(1) 0.93(1) 1.03(1) 1.56(1)

0.432(4) 0.670(4) 0.757(4) 0.799(5) 0.930(6)

0 0.072(3) 0.091(3) 0.111(3) 0.240(3)

a

Values in parentheses indicate one standard deviation in the refined value.

refinements implies that at these moderate loading levels adsorbed Xe atoms begin to condense randomly on the pore surfaces. This observation is consistent with both the leveling off of experimental heats of adsorption in this range and the GCMC simulations, which do not predict well-defined binding sites within the large pores at moderate and high loadings of Xe (Figure 5d,e). Site preferences for Kr determined from XRPD data are similar to those of Xe with slight differences in the refined locations of the pocket and window sites. The initial, strongest binding site is also within the tetrahedral pocket; however, Kr does not occupy the center of the pocket but shifts slightly offcenter (Figure 11, left). Presumably, with an appreciably

Figure 10. Experimental heat of adsorption data with overlaid dose amounts for Xe (left) and Kr (right). 20123



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Figure 11. Binding sites for Kr (pink atoms) in and around one pocket of HKUST-1. View on the left shows a pocket site that is slightly shifted from the center of the pocket (only one of the four symmetry-related pocket sites is shown); view on the right shows the secondary window site with all four possible locations of the pocket site.

those predicted from the GCMC simulations (Figure S20 of the Supporting Information). Adsorbed Ar occupies a site essentially between the window site and the central pocket site first occupied for Kr and Xe (Figure 12). This allows for close contacts with the ligands forming the pocket surface, with Ar− Ar distances (4.02(7) Å) large enough such that four Ar atoms can fit within the pocket at once. To summarize, we find no evidence of any interactions between noble gas atoms and the accessible Cu(II) site. All distinct binding sites are found in and around the small pockets, confirming the assignments inferred from the adsorption data and simulations. These observations provide an important contrast to what has been shown in HKUST-1 for gases with similar sizes and polarizabilities, such as methane, where binding is observed at the Cu(II) sites in addition to the pocket and window sites.16,17

Table 2. Refined SOFs for Kr Sites As a Function of Loading Amountsa dose

Kr adsorbed per Cu

Kr adsorbed per pocket

Kr adsorbed (mmol/g)

pocket site SOF

window site SOF

1 2 3

0.075(1) 0.184(3) 0.372(4)

0.449(8) 1.10(2) 2.23(2)

0.371(6) 0.91(1) 1.84(2)

0.112(2) 0.175(2) 0.242(3)

0 0.100(4) 0.316(5)

a

Values in parentheses indicate one standard deviation in the refined value.

three pocket sites closest to opposite windows. Adsorption at the lower energy window site with increased loadings correlates well with the decline in observed heat of adsorption (Figure 10, right). As with Xe adsorption, XRPD data collected at higher loadings for Kr display an increase in background and decrease in peak intensity that makes structure refinement ambiguous. Assignments of Ar adsorption sites inferred from the experimental adsorption data and GCMC simulations were confirmed by neutron diffraction experiments. The Fourier difference plot obtained from refinement of a data set collected at a loading of 0.86 mmol/g of Ar compared to the bare structure shows adsorbed gas density at locations identical to

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CONCLUSIONS HKUST-1 remains a promising material for Xe/Kr separations. Given the number of studies on both this compound as well as other MOFs that will be compared to it, it is critical to understand the nature of its noble gas adsorption at a fundamental level. Our results show that a combination of

Figure 12. Binding sites for Ar (green atoms) inside pocket windows in HKUST-1. View on the left shows one isolated pocket looking through the window at the four Ar atoms arranged within the pocket; view on the right shows one unit cell viewed down the c-axis to reveal the primary binding site. 20124



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carefully collected gas adsorption isotherms and computationally inexpensive GCMC simulations can provide a complete description of gas uptake in this material, an approach we are currently applying to other systems. The agreement between our experimental data (from both isotherms and calorimetry) and GCMC simulations suggests such calculations represent a valuable screening tool in noble gas adsorption, especially considering their striking correlation with direct structural characterization using X-ray and neutron diffraction data. While the accessible Cu(II) sites dominate adsorption of H2 and other gases, we find no evidence of any interaction of the noble gases with these sites. Preferential adsorption occurs either inside the small tetrahedral pockets or at the windows leading into them. The strong affinity for Xe binding at the pocket site explains the ability of HKUST-1 to efficiently separate Kr/Xe mixtures. HKUST-1 may be particularly effective because of a combination of selectivity from small pockets and easy accessibility of these pockets through large pores.

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ASSOCIATED CONTENT

Adsorption isotherms at all temperatures for all gases, additional information regarding calorimetric experiments and simulations, and detailed Rietveld refinement results for all structures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (702) 895-3753. Notes

The authors declare the following competing financial interest(s): R.Q.S. has a financial interest in NuMat Technologies, a company that is pursuing commercialization of MOFs.

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REFERENCES

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S Supporting Information *

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Article

ACKNOWLEDGMENTS

We thank David Gilley, Jeff Kenvin, and Graham Killip of Micromeritics, Eric Lecher of ColdEdge Technologies, and Eric Knight at University of Nevada, Las Vegas for assistance with our experimental setup, and Matthew Hudson and Rachel Pollock of the National Institute of Standards and Technology Center for Neutron Research and Greg Halder of the Advanced Photon Source (APS) for assistance with powder diffraction experiments. This research is being performed using funding received from the U.S. Department of Energy (DOE) Office of Nuclear Energy’s Nuclear Energy University Programs and from the DOE Office of Basic Energy Sciences (DE-FG0208ER15967). Calorimetric measurements were supported as part of Materials Science for Actinides, an Energy Frontier Research Center funded by the DOE Office of Science, Office of Basic Energy Sciences (DE-SC0001089). Use of the APS, an Office of Science User Facility operated for the U.S. DOE Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC0206CH11357. D.F.J. thanks the Royal Society (U.K.) for a University Research Fellowship. 20125



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